This invention relates to recovery of liquid and gaseous products from subterranean formations containing oil shale, and more particularly, to techniques for providing a gas seal in a lower production level of an in situ oil shale retort.
The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. The term "oil shale" as used in the industry is in fact a misnomer; it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer caller "kerogen" which, upon heating, decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil".
A number of methods which have been proposed for processing oil shale involve either first mining the kerogen-bearing shale and processing the shale on the ground surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, such as U.S. Pat. Nos. 3,661,423, 4,043,595; 4,043,596; 4,043,597; and 4,043,598, which are incorporated herein in this reference. These patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale wherein such formation is fragmented by explosive expansion techniques, forming a fragmented permeable body or mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort.
In forming such a fragmented mass, at least one void is excavated from formation within the retort site, leaving a remaining portion of unfragmented formation within the retort site adjacent the void. Explosive is loaded into blasting holes drilled in the remaining portion of unfragmented formation. The explosive is detonated for explosively expanding the remaining portion of unfragmented formation toward the free face of formation adjacent the void for forming a fragmented permeable mass of formation particles containing oil shale in an in situ oil shale retort.
During retorting, hot retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone in the fragmented mass and introducing an oxygen-supplying gaseous combustion zone feed into the fragmented mass to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen in the combustion zone feed is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. By continued introduction of the combustion zone feed into the fragmented mass, the combustion zone is advanced through the fragmented mass.
The combustion gas and the portion of the combustion zone feed that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone. This heats the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called retorting, in the oil shale. The kerogen decomposes into gaseous and liquid products, including gaseous and liquid hydrocarbon products, and to a residual solid carbonaceous material.
The liquid products and gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. During retorting, the liquid products and a process off gas containing gaseous products pass to a lower level of the fragmented mass. The liquid hydrocarbon products, together with water produced in or added to the retort, are collected at the bottom of the retort. An off gas also is withdrawn from the bottom of the retort. The off gas contains combustion gas, including carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process. The products of retorting are referred to herein as liquid and gaseous products.
The water and shale oil can be separately withdrawn through a production level drift. The process off gas also is withdrawn through the production level drift. The off gas can contain nitrogen, hydrogen, carbon monoxide, carbon dioxide, water vapor, methane, and other hydrocarbons and sulfur compounds, such as hydrogen sulfide. Hydrogen sulfide and carbon monoxide are extremely toxic gases. For this reason, the production level drift is sealed against the passage of off gas from the portion of the drift where the gas is withdrawn, so that workers in adjacent underground workings at the production level are isolated from the off gas collected in the production level drift.
The production level drift can be sealed by a bulkhead placed across the drift. The bulkhead can comprise a steel bulkhead plate secured to a rigid framework. Concrete anchors the peripheral portion of the bulkhead and seals the bulkhead across the cross-sectional area of the drift.
The present invention provides a gas seal for a production level drift wherein the need for a bulkhead sealed across the drift can be eliminated. The invention thus can reduce this cost of forming a gas seal in a production level drift because the materials cost for the conventional bulkhead structure can be avoided, as well as the time and cost required to form the bulkhead in the drift. Gas leakage through a bulkhead is avoided and leakage problems through formation around such a bulkhead are alleviated.